The Comp Sys Workbench has 2 projects that demonstrate interfacing to a MMC card (works just as well if you plug in a SD card) ... FAT formatted, but only supports writing a single file in the root directory. Source code can be downloaded free with a "not for commercial use" license. Complete assembled demo boards are available, as well as empty PCBs. The demo boards each include a SDMMC connector, a MCU, and a standard DB9 RS232 connector.

PIC+MMC interfacing: one version uses a 18LF2620. The "PIC16F876 to MMC Interface Project" version uses a PIC16F876, which has so little internal RAM that the blocks of 512 bytes required by the MMC protocol are buffered in a Ramtron 64kbit FRAM.